The advent and use of body armor has substantially reduced fatalities from explosions, especially soldier fatalities from explosive attacks. Lower mortality rates from primary injuries, such as fragments, however have been accompanied by a significant rise in the incidence of other injuries, such as blast-induced traumatic brain injury (hereinafter “biTBI”). Such injuries can be difficult to diagnose since symptoms can appear long after exposure to a blast, and injured victims often self-report immediately after the blast that they are fine.
It is known that the human body's ability to tolerate increases in external pressure above the ambient pressure depends on (1) the rate of pressure increase; (2) the peak value (i.e. magnitude) of the pressure increase; and (3) the duration of the pressure increase. In general, slow increases in pressure are tolerated well, even for long durations. For example, a scuba diver descending slowly (over many tens of seconds) to 120 feet will experience an additional four atmospheres of external pressure, with no deleterious effects at depth. However, serious injury can occur when the pressure rises rapidly (microseconds or less), as in a blast wave. It is appreciated that a blast wave in air is a rapidly moving pressure wave exceeding many hundreds of meters per second that produces a sudden increase in pressure above the ambient pressure. FIG. 9 shows an amplitude vs. time graph of a typical blast wave in air. The sudden increase (rapid rise time) in pressure that exceeds the ambient pressure, especially one that is induced by a shock or blast wave, is called overpressure. After the blast wave passes a particular location, the blast-induced overpressure decreases slowly (relative to the rise time) from the peak value (magnitude) to values that for a short time fall below the original ambient pressure. The pressure eventually returns to the ambient value long after the blast wave has passed. In FIG. 9, the blast duration is illustrated as the difference in trigger time or TOA between the positive pressure change above ambient pressure and a negative pressure change below ambient pressure.
In general, the greater the magnitude of the blast-induced overpressure and the longer the duration of the blast-induced overpressure, the more severe the biological damage due to the blast wave. For example, a few atmospheres of blast-induced overpressure experienced for a few milliseconds is known to cause severe biological damage. The severity of the problem is compounded because simulations have shown that even small overpressures with rapid rise times can produce significant flexure in the skull (a previously unrecognized/unreported mechanism), which can generate large pressure gradients in the brain that may be a primary mechanism for biTBI).
Diagnosis of biTBI is problematic because precise biological damage thresholds are not currently known, and blast exposure is affected significantly by a blast victim's (e.g. soldier's) local environment. For example, blast exposure in an unconfined space is much less severe than in an enclosed space, or near a wall or interior corner, and can also differ from conditions inside a vehicle. Consequently, it is difficult to determine the severity of the blast wave to which a blast victim has been exposed. This makes determination of biological damage thresholds from field injury data challenging. And even if these thresholds were known, they cannot be used to diagnose biTBI unless the exact blast conditions experienced by a particular individual can be measured. The objective determination of the severity of blast effects requires assessment during the exposure.
What is needed therefore is a helmet blastometer for determining blast conditions that give rise to biTBI, such that the diagnosis of biTBI can be objective rather than subjective.